Congratulations, Blondie!
I have two year old twins and had a very good pregnancy with them. They were born at 35 weeks and did not have to go into Nicu at all.
I was not restricted in regard to exercise by my doctors, they just said to listen to my body. I wasn't able to work out much in the beginning because I had alot of nausea, more so than with my first pregnancy. Once my second trimester rolled around, I was much better able to work out. I didn't do any Cathe workouts, mainly I walked or did toning, yoga, etc. By the third trimester I was rather big, so yoga was my mainstay - when I felt like doing anything. (I was just happy they don't advocate mandatory bedrest anymore for multiple pregnancies - I had a (then) 2 year old to take care of!)
Even though I gained a good bit, I was all babies(!) which put alot of strain on my back. That's why I didn't feel comfortable doing much cardio. I really think you shouldn't get too concerned with the number on the scale, just remember your number one job is to take care of yourself now so you can take care of those babies!
I'm wondering if you had any concerns with your first pregnancy that would explain why your doctor is worried about you carrying the twins?
I wish you the best of luck during your pregnancy and beyond. If you are at all interested, I still have books dealing with multiple pregnancy that you are welcome to - just post here or feel free to email me.
Tricia